Problem: Solve for $r$ : $8 + r = 17$
Subtract $8$ from both sides: $ \begin{eqnarray} \\ 8 + r &=& 17 \\ \\ {-8} && {-8} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{8 + r} &\hphantom{=}& \hphantom{17} \\ r &=& 17 {- 8} \\ \end{eqnarray} $ Simplify. $ r = 9$